ERROR: While executing gem ... (Gem::InstallError)
invalid gem format for c:/ruby/lib/ruby/gems/1.8/cache/activesupport-2.1.0.gem
So here is the solutions:
After doing some sort of R&D i found that you will need to download 'activesupport-2.1.0.gem' from http://rubyforge.org/frs/?group_id=570 site and first install activesupport manually. and then run 'gem install rails' from command prompt.